U.S. patent number 5,606,511 [Application Number 08/368,919] was granted by the patent office on 1997-02-25 for microcontroller with brownout detection.
This patent grant is currently assigned to Microchip Technology Incorporated. Invention is credited to Randy L. Yach.
United States Patent |
5,606,511 |
Yach |
February 25, 1997 |
Microcontroller with brownout detection
Abstract
A microcontroller device is fabricated in a semiconductor
integrated circuit chip to control an external system with which
the device is to be installed in circuit. The device has a CPU, a
program memory for storing program instructions to be implemented
by the CPU, a data memory for storing data including data
pertaining to parameters of the external system to be controlled by
operation of the CPU according to the instructions, and various
peripherals. A brown-out protection circuit monitors the level of
the supply voltage for the IC chip relative to a ground reference
level, to reset the device as protection against its malfunction in
response to reduction of an arithmetic difference between the
supply voltage level and the ground reference level to a value less
than a predetermined threshold operating voltage level. Reset
defines a cessation of operation of the device while maintaining
status quo of implementation of program instructions by the CPU and
data stored in the data memory. A discriminator distinguishes
between a reduction representative of a brown-out event warranting
invoking a reset of the device and a reduction representative of
mere transitory voltage swings commonly occurring in the device
operation not warranting invoking a reset of the device. This
serves to avoid both malfunctions and unnecessary resetting of the
device in control of the external system.
Inventors: |
Yach; Randy L. (Phoenix,
AZ) |
Assignee: |
Microchip Technology
Incorporated (Chandler, AZ)
|
Family
ID: |
23453298 |
Appl.
No.: |
08/368,919 |
Filed: |
January 5, 1995 |
Current U.S.
Class: |
702/64; 714/22;
713/340; 702/190 |
Current CPC
Class: |
H02H
3/247 (20130101) |
Current International
Class: |
H02H
3/24 (20060101); H02H 3/247 (20060101); G01R
019/00 () |
Field of
Search: |
;364/492,483,493,550,579
;395/750,550 ;371/67.1 ;340/660-663 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Trammell; James P.
Attorney, Agent or Firm: Wigman, Cohen, Leitner & Myers,
P.C.
Claims
What is claimed is:
1. In a microcontroller device fabricated in a semiconductor
integrated circuit (IC) chip for controlling an external system
with which the device is to be installed in circuit, and having a
central processing unit (CPU), a program memory for storing program
instructions to be implemented by the CPU, a data memory for
storing data including data pertaining to parameters of the
external system to be controlled by operation of the CPU according
to said instructions, means for providing a supply voltage to
operate the IC chip, said supply voltage being defined by a voltage
level relative to a ground reference level, and brown-out detection
means for monitoring the supply voltage level relative to the
ground reference level to reset said device as protection against
malfunction thereof in response to reduction of an arithmetic
difference between the supply voltage level and the ground
reference level to a value less than a predetermined threshold
operating voltage level, wherein reset defines a cessation of
operation of the device while maintaining status quo of
implementation of program instructions by the CPU and data stored
in the data memory as at the time of occurrence of the reset, the
improvement comprising discrimination means for distinguishing
between a reduction representative of a brown-out event warranting
invoking a reset of said device and a reduction representative of
mere transitory voltage swings commonly occurring in the device
operation not warranting invoking a reset of said device, and means
for suppressing said brown-out detection means from resetting said
device when said reduction of the difference between the supply
voltage level and the ground reference level represents a
transitory voltage swing not warranting invoking a reset, whereby
to avoid both malfunctions and unnecessary resetting of the device
in control of the external system.
2. The improvement of claim 1, wherein said discrimination means
includes filter means for delaying response of said brown-out
detection means to each reduction representative of said commonly
occurring transitory voltage swings, whereby to allow a transitory
voltage swing to expire before restoration of normal device
operating voltage level.
3. The improvement of claim 2, wherein said filter means includes
sample filters coupled to said supply voltage means to derive from
said supply voltage a pair of sample values for establishing, in
response to supply voltage fluctuations relative to one of the
sample values, when a reset should be invoked and, in response to
supply voltage fluctuations relative to the other of the sample
values, when the invoked reset should be terminated.
4. The improvement of claim 3, wherein said filter means further
includes hysteresis means coupled to said sample filters and
adapted to be adjusted to a value related to said pair of sample
values to add a corresponding delay interval to a first point in
time at which said one sample value establishes when a reset should
be invoked.
5. The improvement of claim 4, wherein said brown-out detection
means further includes timer means responsive to a supply voltage
fluctuation at a second point in time at which said other sample
value establishes when a reset should be terminated, for
maintaining said device in reset condition for a predetermined time
interval, and for thereafter restoring device operation at a point
corresponding to the status quo at which the reset was initially
invoked.
6. The improvement of claim 1, wherein said brown-out detection
means further includes timer means responsive to restoration of
normal device operating voltage level following a reset of said
device, for maintaining said device in reset condition for a
predetermined time interval, and for thereafter restoring device
operation from a point corresponding to the status quo at which the
reset was initially invoked.
7. A microcontroller device including a microprocessor, memory and
peripherals for controlling a controlled system, and a source of
supply voltage for operating said microprocessor, memory and
peripherals; further including brown-out protection means for
monitoring the supply voltage to invoke a reset of said
microcontroller device in avoidance of malfunction thereof when the
supply voltage level fluctuates below a predetermined threshold
operating voltage level, wherein reset defines a halt in the
operation of the microcontroller device while freezing status quo
of said microprocessor, memory and peripherals at the point of halt
of the operation, said brown-out protection means including means
for selectively disabling response of said brown-out protection
means to supply voltage level fluctuations, otherwise tending to
invoke a reset of said microcontroller device, which are only
attributable to noise or switching transients normally encountered
in said microcontroller device operation, whereby to avoid both
malfunctions and unnecessary resetting of the microcontroller
device during control of the controlled system.
8. The microcontroller device of claim 7, wherein said selective
disabling means includes delay means for delaying response to
fluctuations attributable to noise or switching transients, whereby
to enhance restoration of normal device operating voltage level
without suspending operation of said microprocessor, memory and
peripherals.
9. The microcontroller device of claim 8, wherein said delay means
includes sampling means responsive to said supply voltage to derive
therefrom a plurality of sample values for establishing different
levels of supply voltage fluctuations to invoke a reset and to
terminate a reset.
10. The microcontroller device of claim 9, wherein said sampling
means further includes hysteresis means electrically connected to
said sampling means to displace both of said sample values by a
variable amount to correspondingly delay invocation of a reset.
11. The microcontroller device of claim 10, wherein said brown-out
protection means further includes timer means responsive to one of
said different levels of supply voltage fluctuation to terminate a
reset, for maintaining said microcontroller device in reset
condition for a predetermined time interval, and for thereafter
restoring device operation at a point corresponding to the status
quo at which operation was halted.
12. In a microcontroller device fabricated in a semiconductor
integrated circuit (IC) chip for controlling an external system
with which the device is to be installed in circuit, and having a
central processing unit (CPU), a program memory for storing program
instructions to be implemented by the CPU, a data memory for
storing data including data pertaining to parameters of the
external system to be controlled by operation of the CPU according
to said instructions, means for providing a supply voltage to
operate the IC chip, said supply voltage being defined by a voltage
level relative to a ground reference level; a device-implemented
method of brown-out protection comprising the steps of monitoring
the supply voltage level relative to the ground reference level to
reset said device as protection against malfunction thereof in
response to reduction of an arithmetic difference between the
supply voltage level and the ground reference level to a value less
than a predetermined threshold operating voltage level, wherein
reset defines a cessation of operation of the device while
maintaining status quo of implementation of program instructions by
the CPU and data stored in the data memory; discriminating between
a reduction representative of a brown-out event warranting invoking
a reset of said device and a reduction representative of mere
transitory voltage swings commonly occurring in the device
operation not warranting invoking a reset of said device, and
suppressing a reset of said device when the reduction of the
difference between the supply voltage level and the ground
reference level represents a transitory voltage swing not
warranting invoking a reset, whereby to avoid both malfunctions and
unnecessary resetting of the device in control of the external
system.
13. The method of claim 12, wherein the step of discriminating
includes delaying response to each reduction representative of said
commonly occurring transitory voltage swings, whereby to allow a
transitory voltage swing to expire before a return to normal device
operating voltage level.
14. The method of claim 13, wherein the step of delaying includes
deriving from said supply voltage a pair of sample values for
establishing, in response to supply voltage fluctuations relative
to one of the sample values, when a reset should be invoked and, in
response to supply voltage fluctuations relative to the other of
the sample values, when the invoked reset should be terminated.
15. The method of claim 14, wherein the step of delaying further
includes using hysteresis to add a corresponding delay interval to
a first point in time at which said one sample value establishes
when a reset should be invoked.
16. The method of claim 15, further including the step, in response
to a supply voltage fluctuation at a second point in time at which
said other sample value establishes when a reset should be
terminated, of maintaining said device in reset condition for a
predetermined time interval, and thereafter restoring device
operation at a point corresponding to the status quo at which the
reset was initially invoked.
17. The method of claim 12, including responding to restoration of
normal device operating voltage level following a reset of said
device, for maintaining said device in reset condition for a
predetermined time interval, and thereafter restoring device
operation from a point corresponding to the status quo at which the
reset was initially invoked.
18. A microcontroller device with a power supply and a brown-out
detection and correction circuit for resetting the device, said
brown-out circuit comprising a voltage reference for providing a
precise reference level; a dual comparator for comparing a pair of
inputs; means supplying the voltage reference level as a first
input to said comparator; means supplying a filtered sample level
of the power supply as a second input to said comparator and for
providing hysteresis and preventing oscillation of the brown-out
circuit; and signal filter supply tracking means for determining
from a sampled level of the power supply whether deviations of the
power supply level from said voltage reference level are of a type
to generate a reset.
19. The microcontroller of claim 18, wherein said signal filter
supply tracking means includes sensor means for detecting the
magnitude and frequency of fluctuations of the power supply for
enabling the brown-out circuit to promptly reset the device when a
fluctuation drops significantly below the voltage reference level,
and for delaying response of the brown-out circuit when a
fluctuation is relatively shallow and at relatively high frequency
to inhibit a reset of the device, whereby to suppress resetting the
device as a consequence of noise or other minor perturbations
alone.
20. The microcontroller of claim 18, wherein said means supplying a
filtered sample level of the power supply is responsive to a reset
of the device for providing hysteresis by establishing a pair of
trip points for comparison with excursions of the power supply
level, to eliminate oscillations of the brown-out circuit, and for
holding the device in reset until the power supply level returns to
at least the voltage reference level and for a predetermined time
interval thereafter.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to semiconductor devices,
and more particularly to a semiconductor microprocessor or
microcontroller with protection against low supply voltage.
Low supply voltage on a semiconductor microcontroller chip
(device), that is, a supply voltage that falls below the minimum
acceptable value or level necessary for proper operation of the
chip or end system (i.e., necessary to avoid a malfunction), can be
especially serious for sensitive applications, such as an
automotive anti-lock braking system. For that reason,
microcontroller chips have typically been provided with `brown-out
protection`--protection against not a complete loss of power but a
reduction in power supply level sufficient to cause malfunction of
the circuit or system. Applications that require brown-out
protection generally are systems with a controlled power supply and
not battery driven.
Theoretically, brown-out protection should reset the
microcontroller chip or system at any time that V.sub.DD falls
below the minimum acceptable value. However, this minimum V.sub.DD
value will depend upon the operating frequency of the device or
other system constraints. For example, the critical value
(V.sub.DDmin) may be 4.0 volts (V) at 20 megahertz (MHz) operation,
and only 1.8 V at 4 MHz.
For that and other reasons, brown-out requirements may be qualified
by market considerations, i.e., the specific application in which
the device is to be used. Applications that require brown-out
protection fall into many categories, such as appliances,
industrial control, and automotive applications. Operational
(plugged-in) appliances encounter brown-out situations during line
voltage dip. Peripheral voltage sensitive elements will malfunction
at that time even though the appliance may not be turned on for its
primary function. Automotive electronics encounter brown-out
situations when the ignition is turned on. For example, the
automobile radio may be turned on by the ignition key just as the
starter is engaged, causing the power supply to drop. The affected
device must reset or it may be left in an unstable state.
Brown-out is not applicable in low voltage battery applications
where power consumption, operating frequency, and noise are at a
relative minimum.
One may then define brown-out for the situation in which V.sub.DD
falls to or below a predetermined voltage threshold level. Because
microcontrollers operate at high speeds, it is important to also
distinguish a real brown-out from normal I/O switching noise which
may last as long as 200 nanoseconds (ns). In the circumstances
noted above, for a 5 V supply, for example, if V.sub.DD falls below
a level of 4.0 V for more than 200 ns, then brown-out detect should
trigger to reset the chip.
Problems arise, however, in situations of relatively shallow dips
of the supply voltage below the specified voltage threshold level,
whether as an isolated excursion or a sequence of slight excursions
of V.sub.DD below and then back above the threshold. In such
instances, whether attributable to noise, I/O switching, or other
line perturbations, the brown-out detection circuit typically
initiates an immediate reset of the microcontroller chip on the
first occasion and, if there is no further recurrence within a set
period, reestablishes operation of the chip promptly after the set
period expires. This, of course, results in disruption of the
operation of the microcontroller when there is no need for a reset.
Also, if V.sub.DD drops to the trip voltage and remains there for a
lengthy period, the circuit may oscillate with attendant unstable
operation.
It is a principal object of the present invention to provide a
brown-out detection circuit which is capable of analyzing the
nature of supply voltage dips, and of applying predetermined
criteria and hysteresis to the determination of whether or not a
reset should be initiated each time the supply voltage drops below
a threshold level.
SUMMARY OF THE INVENTION
Ideally, in microcontroller applications, the brown-out protection
should reset the microcontroller whenever the supply voltage drops
below a predetermined value. Brown-out typically applies to
applications in which the microcontroller is used with a controlled
power supply rather than a battery, so the voltage range of
interest is the supply system voltage .+-.a delta value.
The present invention employs a three-section CMOS-fabricated
brown-out detection and correction circuit as a portion of the
microcontroller chip. One section of the brown-out circuit is a
voltage reference circuit constituting a precise voltage reference.
A second section of the circuit is a dual comparator circuit which
has the voltage reference as one input and filtered power supply
sample level as another input. The dual comparator is responsible
for the circuit hysteresis and keeps the circuit from oscillating.
The third section comprises a signal filter and supply tracking
mechanism used to sample the supply level and to determine whether
the brown-out is of a type to generate a reset.
According to the invention, this final section of the brown-out
circuit includes a built-in sensor which serves to filter out the
depth of the V.sub.DD spikes with the frequency. If a spike is
below the specified threshold level and very deep, the brown-out
detect circuit will cause the chip to be reset immediately, subject
to the power-up timer timeout interval. On the other hand, if the
spike is relatively shallow, or at a very high frequency, the
brown-out detect circuit will respond more slowly and consequently
may not reset the chip. This is highly desirable in an application
because it precludes a resetting of the device as a consequence of
noise or other minor perturbations alone. In essence, the circuit
filters out normal noise, but reacts quickly to trigger reset if a
true brown-out condition is detected.
Once in reset, the circuit uses hysteresis to eliminate
oscillations and holds the device in reset until the power supply
returns to normal operating voltage level, at which time a power-up
timer is invoked to maintain the reset condition for a fixed time
interval. Hysteresis greatly assists the dual comparator to
distinguish a true brown-out event from normal noise or other
slight perturbations, and is achieved by dual sampling of the power
supply and establishing a pair of trip points for comparison with
excursions of the supply voltage level.
In a preferred embodiment, the microcontroller device is fabricated
in a semiconductor integrated circuit (IC) chip to control an
external system with which the device is to be installed in
circuit. The microcontroller includes a central processing unit
(CPU), a program memory for storing program instructions to be
implemented by the CPU, a data memory for storing data including
data pertaining to parameters of the external system to be
controlled by operation of the CPU according to the instructions,
and various peripherals. A brown-out protection circuit monitors
the chip supply voltage level relative to a ground reference level,
and operates to reset the device to protect it against malfunction
in the event of a reduction of the difference between the supply
voltage and ground reference levels to a value less than a
predetermined threshold operating voltage level. Reset causes
operation of the device to be halted while the status quo of
implementation of program instructions by the CPU and data stored
in the data memory is maintained as it was at the time of
occurrence of the reset.
Discrimination means of the brown-out protection circuit
distinguishes between a reduction of the aforesaid voltage
difference representative of a brown-out event warranting invoking
a reset of the device, and a reduction representative of mere
transitory voltage swings commonly occurring in the device
operation not warranting invoking a reset of the device.
Consequently, both malfunctions and unnecessary resetting of the
device during performance of its control function on the external
system are avoided. The discrimination means includes filter means
for delaying response of the brown-out protection circuit to each
reduction representative of the commonly occurring transitory
voltage swings, so as to allow a transitory voltage swing to expire
before restoration of normal device operating voltage level.
The filter means includes sample filters that derive from the chip
supply voltage a pair of sample values for establishing, in
response to supply voltage fluctuations relative to the lower of
the sample values, when a reset should be invoked and, in response
to supply voltage fluctuations relative to the higher of the sample
values, when the invoked reset should be terminated. The dual
sample also provides hysteresis means coupled to the sample filters
and adapted to be adjusted to a value related to the pair of sample
values to remove oscillations in circuit response. A power-up timer
responds to a supply voltage fluctuation at a second point in time,
at which the other sample value establishes when a reset should be
terminated, to maintain the device in reset condition for a
predetermined time interval, and thereafter restores device
operation at a point corresponding to the status quo at which the
reset was initially invoked.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and still further objects, features, aspect and attendant
advantages of the invention will become apparent from a
consideration of the following detailed description of the
presently contemplated best mode of practicing the invention, with
reference to a preferred embodiment, taken in conjunction with the
accompanying drawings in which:
FIG. 1 is a simplified block diagram of a microcontroller chip with
certain peripheral components including a brown-out detector;
FIG. 2 is a simplified block diagram of a brown-out detector
circuit for use in the microcontroller of FIG. 1, according to the
invention;
FIGS. 3A and 3B are graphs illustrating the internal operation of
the power monitor circuit in the microcontroller device of FIG.
1;
FIG. 4 is a graph illustrating a deep V.sub.DD drop and immediate
reset;
FIGS. 5A and 5B are a simplified block diagram and a somewhat more
detailed circuit schematic diagram, respectively, of a signal
filter and sample circuit used in the brown-out detector of the
invention;
FIG. 6 is a graph illustrating normal ground noise with no reset;
and
FIG. 7 is a graph illustrating an increase in ground reference
level that causes a reset.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT AND METHOD
Referring to FIG. 1, an exemplary microcontroller device employing
the principles of brown-out protection according to the present
invention includes Central Processing Unit (CPU) 10, program memory
12, data memory 13, and various peripherals shown as a block 15.
Power is supplied to the system from a source designated as
V.sub.DD. A power monitor 16, or brown-out detector, is connected
to the power supply to assess the instantaneous level of V.sub.DD,
and, if it falls below a predetermined minimum acceptable value, to
detect that situation and thereupon reset the device. As pointed
out above, however, the level of V.sub.DD depends in part on the
frequency of I/O switching characteristics of the device.
Power monitor 16, therefore, is adapted to respond to a situation
in which V.sub.DD drops below the designated minimum acceptable
level for a predetermined time interval that depends on normal
acceptable noise frequencies. Although the brown-out detection
should reset the chip in those circumstances, it is important to
avoid resetting where perturbations in V.sub.DD that may be
attributable to noise or I/O switching drive the voltage level
momentarily below and then back up to or above this threshold
value. The power monitor is specifically designed to delay any
reset of the microcontroller device that may unnecessarily cause
disruption or instability of system operation.
As shown in FIG. 2, the power monitor is a brown-out detection and
correction circuit that comprises three sections, one of which is a
precise voltage reference generator 20. The precise reference
voltage is set at a stable voltage output, applied to a comparator
circuit 21. The comparator and associated logic circuitry are
designed to compare the precise reference voltage output of
generator 20 against an equalized sample of the instantaneous level
of V.sub.DD at any given time. However, the comparison must be
undertaken and accomplished in a way that will avoid the
aforementioned unnecessary and potentially failure-causing
disruptions or oscillations of system operation. This is
particularly important in applications where personal injury or
property damage may be a consequence of such disruption or
instability, such as with anti-lock braking systems (ABS) in
automobiles.
A power sample and filter circuit 23 is employed to measure and
periodically sample the V.sub.DD level, but also to filter out
spikes which take V.sub.DD below the reference level, and to
perform the filter function according to the I/O switching
frequency characteristics of the microcontroller device operation.
In operation, if a deep drop (negative spike) in V.sub.DD occurs in
a voltage sample, the comparator logic of circuit 21 reacts to the
deviation from the reference voltage to reset the microcontroller
with an output R1, based on a pair of sample voltages V1 and V2
from circuit 23. A circuit 24 to which output R1 is applied
includes a power-up timer with a timeout interval selected as
necessary for system stabilization. In instances where only a
shallow spike or a series of spikes occurring at high frequency
(relative to the microcontroller I/O switching characteristics) is
detected, the filtering function performed by circuit 23 produces a
variably slower response of the voltages V1 and V2, so that the
triggering of a reset is much less likely than in the sudden deep
spike circumstance.
Some examples of this operation are illustrated in FIGS. 3A and 3B.
For oscillation protection, a detection range is selected having
upper and lower voltage threshold limits V1 and V2, respectively,
based on a delta difference from a threshold level displaced a
certain voltage from the normal voltage level of V.sub.DD. A
reference voltage VRef is used for comparison. For a 5 V supply,
for example, a permissible drop may be 1.0 V with a delta of
.+-.0.2 V. V1, then, is selected to cross VRef when V.sub.DD =4.2 V
and V2 is selected to cross VRef when V.sub.DD =3.8 V. Based on
normal high frequency noise characteristics for a microcontroller,
the predetermined delay time interval of 200 ns to initiate reset
of the chip is imposed by the filter starting from the time
V.sub.DD falls below VRef until V1 drops below VRef.
The chip is then driven into reset and held there until V.sub.DD
returns to a level indicative of restoration of power. At that
point, the power-up timer of the brown-out detector circuit is
invoked to maintain the chip in reset for a time interval of 72
milliseconds (ms). In the example of FIG. 3A, that occurrence is
measured from the point in time that V.sub.DD achieves a level such
that V2 again exceeds VRef.
On the other hand, the example of FIG. 3B illustrates that the
V1/V2 delay introduced by the filter results in fast disturbances
being ignored. In this instance, the chip does not undergo
reset.
In the circumstances shown in FIG. 4, a relatively deep spike in
V.sub.DD results in an immediate reset of the microcontroller chip.
The reset condition is then maintained for a 72 ms timeout interval
after the power is restored to a level corresponding to that of
FIG. 3A in which V2 exceeds VRef.
The filter network of the sample and filter circuit 23 is
illustrated in block diagrammatic form in FIG. 5A. A pair of supply
filters 30 and 31 provide the levels V1 and V2, respectively, to
the comparator circuit 21 (FIG. 2). The V.sub.DD supply voltage
relative to ground reference level is used by supply filter 30 to
derive voltage level V1, and another voltage level V2 is derived
from the combination of supply filter 31 and hysteresis filter 33.
In the circuit schematic of FIG. 5B, voltage V1 is derived between
electrical resistances 36 and 37 of three resistances (the third
being resistance 38) in series between V.sub.DD and ground, and a
circuit node of the hysteresis filter at a capacitance 39 to
ground. Voltage level V2 is taken from a node between resistances
37 and 38 from which another capacitance 40 of the hysteresis
filter is grounded. The circuits in which the two capacitances are
located comprise a pair of matched RC filters. It will be apparent,
then, that voltages V1 and V2 vary with each fluctuation of
V.sub.DD.
Because the brown-out detector (power monitor) 16 monitors the
power supply V.sub.DD, it also automatically monitors the ground
reference level, as voltages are always measured between two nodes.
This can be undesirable since normal ground noise might cause a
reset of the microcontroller device, but sample and filter circuit
23 prevents this from occurring because the matched RC filters
connected to ground serve, among other things, to filter out the
normal ground noise. Thus, the same filtering mechanism for the
supply side is employed on the ground side (i.e., the circuit of
FIG. 5A (or 5B) affects both sides in the same way) so that the
ground level is also sampled and may reset the chip only if the
difference in level between V.sub.DD and ground is less than the
trip voltage that would initiate a reset (i.e., if V.sub.DD
-Gnd.ltoreq.V.sub.bo, where V.sub.bo is the brown-out voltage).
FIG. 6 illustrates the situation in which normal ground noise is
occurring, but because the difference between V.sub.DD and ground
is not less than V.sub.bo, there is no reset. In FIG. 7, on the
other hand, there is a point at which their difference becomes
equal to (and ultimately less than) the trip voltage, and at that
point a reset condition occurs.
When the microcontroller chip is in the reset condition, the
circuit of FIG. 5A operates, by virtue of the hysteresis filter 33,
to dampen out oscillations and to maintain the chip in the reset
condition until V.sub.DD is restored to its normal operating level
(i.e., V.sub.DD -Gnd>V.sub.bo). At that point, the power-up
timer 24 (FIG. 2) in the brown-out detector circuit is invoked to
initiate its fixed time-out interval, to hold the microcontroller
chip in reset. When that interval expires (i.e., the power-up timer
times out), the chip is removed from reset. Thus, the application
of hysteresis serves to differentiate the occurrence of an actual
brown-out event from the presence of normal noise or switching
transients on the power supply (or ground reference).
The hysteresis is derived from sampling V.sub.DD by the two supply
filters, which creates trip points for initiating an internal reset
(when V.sub.DD drops so that V1<VRef) and initiating the
power-up timer interval (when V.sub.DD rises so that V2>VRef).
Consequently, the trip points equate to: V1>V.sub.bo >V2. In
the illustrative embodiment of the invention, if the brown-out
voltage V.sub.bo is approximately 4.0 V, the hysteresis level
(which is not greater than V2-V1) may have a value of, say, 0.1
V.
Although the best mode presently contemplated for practicing the
invention has been described in terms of a presently preferred
embodiment and method, it will be apparent to those skilled in the
relevant field that variations and modifications may readily be
implemented without departing from the true spirit and scope of the
invention. Accordingly, it is intended that the invention should be
limited only by the appended claims and the pertinent rules of
applicable law.
* * * * *